8.2 Draw Control

In many machines, the draw is a fixed value. The machine designers determine the draw by selecting gear ratios and roller diameters. Many slitter-rewinders and line-shafted presses drive two or more rollers with one motor. Each driven roller’s rpm is determined by motor speed and the gear ratio of the motor to the rollers. The rpm turns in to a surface speed depending on the roller’s circumference.

Roller speed is the roller’s circumference times the roller’s rotational rate (in RPMs or radians/s). Either of these two variables can be used to create the draw. In some machines, two or more rollers are driven at the same rotational rate but have an engineered variation in diameter and circumference to create the desired speed differential and draw. Other machines will use uniform roller diameters, but vary the rotational rates by motor speed or gearing to create the draw. Many machines use a combination of diameter and gearing to create the draw. In any case, understand that your web, especially high modulus and thick webs, are quite sensitive to even small draw ratios, so carefully calculate and ensure draws are calibrated properly in all applications.

Some machines have a programmable draw, allowing you to dial in the ratio or percent draw between driven sections. Closed-loop tension control using pacer and follower driven rollers also will use draw, but the draw will be moving up and down to satisfy the tension trim control loop.

Draw is a machine characteristic. Often it is confused with strain, which is a web property. Draw and strain both can be described in percent, but where strain is always relative to zero strain; draw can be relative to any initial speed. An untensioned 250 mm (10-in.) web sample stretched 2.5 mm (0.1 in.) has a strain of 1%. Given two driven rollers, if the first is driven at 100 m/min (300 fpm) and the second at 101 m/min (303 fpm), this is a draw of 1%. For larger draws, it is more common to talk about a draw ratio. If two sections are driven at 30 and 90 m/min (100 and 300 fpm) such as in length orienting film, the draw ratio is 3:1.